Direct current motor drive

ABSTRACT

A DC motor maintains a constant preset speed under changes of load, the preset speed being variable manually. The system includes an AC source, with a conventional bridge rectifier using thyristors and diodes, with means for controlling the phase of firing the thyristors depending on the motor speed in comparison with a speed setting. A dynamic braking system is provided to cause acceptably fast deceleration when a reduction in speed is called for.

i United States Patent Ottoson Oct. 28, 1975 DIRECT CURRENT MOTOR DRIVE3,8l2.409 5/1974 Dinger 318/310 [75] Inventor: Allen E. ()ttoson,Westboro, Mass.

Primary Examiner-Robert K. Schaefer [73] Asslgnee: Corporatlon,weslbol'o Assistant Examiner-W. E. Duncanson, .11.

Mass Attorney, Agent, or Firm--Kenway & Jenney [22] Filed: Mar. 28, 1974{21 Appl. No.: 455,669 {57] ABSTRACT A DC motor maintains a constantpreset speed under 2 u changes of load, the preset speed being variablemanu- EZ 363353; ally. The system includes an AC source, with a con-[58] Field 3H) 3H ventional bridge rectifier using thyristors anddiodes,

""""" 8/33 with means for controlling the phase of firing the thyristorsdepending on the motor speed in comparison [56] References Cited with aspeed setting. A dynamic braking system is provided to cause acceptablyfast deceleration when a re- UNITED STATES PATENTS duction in speed iscalled for. 3,497,786 2/1970 Lombardo 318/331 x 3,792.330 2/1974 Ottoson318/269 4 Claims, 10 Dl'flWIng Figures I .70 CONTROL 31:22: 1? CONTROLCIRCUIT 174\ L L FROM Q US. Patent Oct. 28, 1975 Sheet 1 of4 FIG. I

I60 50 no DYNAMIC CONTROL BRAKING CIRCUIT CONTROL CIRCUIT /38 I74\ c I8230 32 m4 we FROM 2 4 5:44 :90

U.S. Patent Oct. 28, 1975 Sheet 2 of4 3,916,276

FIG. 2 2O 54 24 4 22 sa es O REFERENCE ARMATURE AND SPEED SETTING 53 56FEEDBACK TACHOMETER FIG. 3

90 H6. 4 k W q V82 FIG. 5 g

U.S. Patent 0a. 28, 1975 Sheet 3 of4 3,916,276

FIG. 6 9 J 1;

|o21r m $80 g n4 o E 1041i 6 78 Ire n I06 T TO 68 TO 1 FIG. 7

FIG. 8

SCR 3O SCR 32 GATE GATE U.S. Patent Oct. 28, 1975 Sheet 4 of4 3,916,276

9 TO we GATE nae I72 To ea I m3 |s7 E To Gate I I68 To Connection 7 of78 and 80 I92 C "Nv l I FIG. I0 T 50 44 DIRECT CURRENT MOTOR DRIVEBACKGROUND OF THE INVENTION DC motors driven from rectified AC sourcesthrough thyristors (silicon controlled rectifiers) have presenteddifficulties in variable-speed operation because of discontinuities incurrent supply to the motor. A measure of the effect of discontinuitiesin the armature current is afforded by the form factor which is theratio of rrns to average current. My prior US. Pat. No. 3,792,330, datedFeb. l2, I974, issued on an application Ser. No. 288,900 filed Sept. 14,1972, describes an improved system, utilizing a substantially constantcurrent source, with thyristors used as current diverters to control thedirect current supplied to the motor.

SUMMARY OF THE INVENTION According to the invention, furtherimprovements are provided, looking toward accurate speed control,precise firing of the thyristors and simple and effective commutation ofthe thyristors, without damage to the thyristors or other parts of thesystem.

An improved dynamic braking system is provided to dissipate the kineticenergy of the armature and cause rapid deceleration from a high to alower set speed, by activation of a thyristor when slow-down is calledfor and precise commutation thereof when the speed reaches the properlevel.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG, 1 is a diagram of the preferred embodiment of the invention;

FIG, 2 is a block diagram to illustrate the speed control;

FIG 3 is a diagram showing the preferred speed control circuitry;

FIG. 4 is a diagram of the phase control for the thyristors;

FIG. 5 is a vector diagram for the control circuit of FIG. 4;

FIG. 6 is a diagram of the preferred means for obtaining variableresistance for phase control;

FIG. 7 is a diagram of the circuit for controlling the firing of thethyristors,

FIG, 8 is a timing diagram;

FIG. 9 is a diagram illustrating the control of a dynamic brakingsystem; and

FIG. I is a diagram of a modified system.

GENERAL DESCRIPTION As shown in FIG. 1, the AC supply lines are at 20,21. One of the lines (20) leads through an inductive reactor or choke 22to a conventional controlled rectifier circuit 24 comprising two diodes26, 28 and two thyristors (silicon controlled rectifiers hereinaftertermed SCRs) 30 and 32, having control gates 34 and 36 respectively. Thediodes and SCRs are arranged as a bridge, with the input power leadsconnected to the junctions a and b, while the junction 0 between the twoSCRs is connected by a lead 38 to the positive terminal of the armature40 of the DC motor. Thejunction d between the diodes is connected by alead 42 to the negative terminal of the armature. An armature filtercapacitor 44 is connected across the armature terminals.

The motor field 46 is excited from the mains through a rectifier 48shown as a conventional fullwave rectifier in the form of a four-diodebridge.

The supply of current to the armature is controlled by gating of theSCRs 30 and 32. Thus, if the SCRs are turned continuously on, the bridge48 acts as a fullwave rectifier to pass maximum current to the motorarmature and armature filter capacitor. When the SCRs are turned offcurrent flows from the armature filter capacitor to the motor armature.By use of the control circuit, shown as a block 50 in FIG. 1, to belater described in detail, the SCRs may be turned on at intermediatetimes in their halfcycles to vary the current passed to the armature andarmature filter capacitor. At some point in a positive halfcycle, thecurrent path is from line 20, through choke 22, junction 0, SCR 30, lead38, armature 40 and capacitor 44, lead 42, diode 28 and junction b tomain line 21. On a negative half-cycle SCR 30 is off and SCR 32 startsto conduct at a controlled time, so that a DC current path leads fromjunction b through SCR 32 and diode 26 to junction a. During the part ofthe cycle when SCRs 30 and 32 are not on, current flows from capacitor44 to armature 40.

CONTROL CIRCUIT FOR SCRS The control circuit 50 is a phase-controlnetwork to control the times at which the SCRs 30 and 32 are turned on.The network is in general similar to that described in myabove-mentioned patent, but because of some differences it will now bedescribed in detail.

A block diagram of the control system 50 appears in FIG. 2. The phasecontrol according to the invention is carried out by a feedback loop 53responsive to a speed setting and to the actual speed of the motor, asindicated by the block 54. The speed setting is a manual setting fordesired speed, by which a reference" voltage is generated, and thisreference voltage is com pared to a voltage proportional to the motorspeed, this speed voltage being conveniently generated by a tachometer56. The reference voltage is supplied by any suitable DC source, shownas a battery 58 in FIG. 3. (Alternatively, rectified AC may be used,with provision for constant DC voltage, as with a Zener diode.)

The reference voltage at the terminals e and fmay conveniently be about[0 volts, A connection is made from the positive terminal 2 to apotentiometer 66 and thence to the negative terminalf. The slider 68 ofthe potentiometer at voltage V, above the negative terminal f is amanual setting corresponding to a desired speed of the motor.

There is a potential difference between the slider 68 and poiotf. Thepotential difference between the slider and pointfis V and thispotential difference is called the reference voltage. This referencevoltage opposes a voltage proportional to the actual speed of the motor,which voltage may be obtained from a tachometer 56 driven by the motor.(In a modified form of the invention to be described later, the speedvoltage may be taken from the armature terminals, with compensation forthe IR drop in the armature.)

As shown in FIG. 3 a connection is made from the slider 68 throughresistors 78 and 80 and the tachometer terminals to the pointf. Thiscircuit carries a current designated the error current I,., and thevoltage across the resistor 78 is an error voltage E,.. This errorvoltage is where the resistance values of the resistors 78 and 80 areindicated by R with appropriate subscripts, V is the reference voltageas above defined and V is the tachometer output voltage.

The error voltage E, is therefore variable. The system parameters arechosen so that the reference voltage V is always greater than the speedvoltage V under stable operating conditions, and the error current I,then flows in the direction indicated by the arrow 1,. A certain valueof E corresponds to the condition of the actual speed being equal to theset speed. If E is higher or lower than that value, the SCRs 30 and 32are fired at earlier or later times in their half-cycles, to increase ordecrease the current supplied to the motor armature.

The manner in which the error voltage is used to control the gates ofthe SCRs to time the firing thereof is by a phase control circuit, whichin general principle is of the usual resistance-capacitance type. InFIg. 4, a variable resistor is shown diagrammatically at 82 withterminals 3 and h. The resistor and a capacitor 84 are connected inseries across the secondary of a transformer 86. Between g at thejunction of the resistorcapacitor and a center tap k of the secondary isa circuit comprising a resistor 88 and the primary of a transformer 90,across which is a capacitor 92. The secondaries of the transformer 90are used to control the transmission of firing pulses to the gates ofthe SCRs, as will presently appear.

The vector diagram for the circuit of FIG. 4 appears in FIG. 5. Sincethe capacitor and variable resistor carry substantially the same current(the current through the 3-1: path being limited by the resistor 88),the voltages V and V across the resistor and capacitor are in quadratureand the locus of the ends of their vectors is a semicircle, whereby thevoltage between 3 and k is of constant magnitude but of varying phase.As the resistance of the resistor 82 is increased the phase of V isretarded.

According to the invention the resistor 82 is varied in accordance withthe error voltage. The lower the error voltage, the higher will be theresistance, and the more will the phase of V,,, be retarded, so thatfiring of the SCR's will occur at later instants in their respectivehalf cycles. the later the firing the less will be energy delivered tothe armature.

Although any suitable arrangement for varying the resistor 82 inaccordance with the error voltage may be used, the preferred form of thephase-control resistor is obtained by use of a transistor and fourdiodes connected as shown in FIG. 6. The four diodes designated 102,I04, 106 and 108 are connected as a bridge, the input terminals of whichare the terminals g and h of the phase-control resistor. The otherjunctions of the bridge are p and q. The error voltage across theresistor 78 is applied between the junction p and the base of atransistor 114 of which the emitter in series with a resistor 116 isconnected to p, while the collector is con nected to q, thetransistor-resistor combination being in the emitter-followerconnection. The junctions g and h of the bridge constitute the similarlydesignated terminals of the resistor 82 shown in Flg. 4. For comparisonwith FIG. 3, the resistor 80, the tachometer terminals and the errorcurrent I are shown in FIG. 6.

By the arrangement of HG. 6, the baseemitter voltage of the transistoris the error voltage E The effective resistance of the circuit betweenthe terminals g and h is therefore governed by the control voltage onthe transistor. The higher the error voltage the more transistor currentwill flow and hence the lower the effective resistance of the paththrough the transistor will be. The diodes allow a unidirectionalcurrent through the transistor with an alternating voltage between theterminals g and h. On one half-cycle current flows from g and it throughdiode 102, transistor 114, resistor 116 and diode 106, and on the nexthalf-cycle, the flow is from h to 3 through diodes 108 and 104.

Noting that changes in current through the transistor 114 result inchanges of effective resistance between the terminals g and h, it willbe seen from FIG. 4 that these changes result in changes of phase of thevoltage across the primary of transformer 90. The phase of the voltageacross the primary of transformer may be used in any suitable orwell-known manner to control the firing angle of the SCRs 30 and 32.However, for purposes of the present invention, a special phase-controlcircuit is preferably used, as shown in FIG. 7.

The primary 90 of the phase control transformer, previously described inconnection with FIG. 4, is shown in FIG. 7. A secondary 118 oftransformer 90 with a capacitor 1 19 across it connects through a diode120 to a parallel resistor 121 and a series capacitor 122 which appliescontrol voltage to the base of a transistor 124.

A diode 141 is connected between the emitter and base of the transistor124. At the other side of the figure is shown another secondary 126 ofthe transformer 90 likewise arranged through a similar circuit to applya control voltage to the base of a transistor 128. These transistorcircuits are powered by a full-wave rectifier circuit including atransformer 130 connected to the line and having a center-tappedsecondary leading through diodes 132 and 134 to a lead 135 connected tothe collector terminals of the transistors 124 and 128, whichtransistors are connected through resistors 136 and 138 respectively tothe center-tap lead 139. A filter capacitor 140 is connected between thepositive lead 135 and the center-tap lead 139. The circuitry associatedwith the transistor 128 is identical with that for the transistor 124,as shown in FIG. 7, and the detailed description is not repeated.

The center-tap lead 139 has a terminal 142 which is connected to thejunction c of the cathodes of the SCRs 30 and 32 of FIG. 1.

In FIG. 7 the junction of the emitter of transistor 124 and the resistor136 is connected through a resistor 146 to a terminal 147, and thecorresponding junction for transistor 128 is connected through aresistor 148 with a terminal 149. The terminals 147 and 149 areconnected to the gates 34 and 36 of the SCRs 30 and 32 respectively,whereby the voltages of these terminals with respect to the terminal 142constitute the gate-tocathode voltages of the respective SCRs. The meansby which gating pulses are transmitted at proper times to the SCRs aredescribed as follows: On a rising part of the sine wave of the voltageof the secondary 118, applied across capacitor 119, the diode 120conducts and turns the transistor 124 on. The transistor conductscurrent in saturation until about the peak of the sine wave, so that thevoltage across resistor 136 is a flat-topped pulse about 70 long. As thevoltage across the secondary 118 declines from its peak the capacitor122 discharges through resistor 121 and diode 141, turning offtransistor 124 and ending the pulse. The capacitor 122 absorbs thedifference between the secondary voltage and the base-emitter voltage oftransistor 124. A similar action occurs in the circuit associated withtransistor 128.

The timing diagram for the phase control circuit is presented in FIG. 8.In the first part of the first halfcycle shown in the diagram the SCRsare turned off. At some phase angle a pulse is applied from the controlcircuit to the gate of SCR 30, thereby causing armature current to flowthrough SCR and diode 28. Current continues through this circuit untilthe total current passes through zero, at which time the SCR 30 isturned off. In the ensuing negative half-cycle no current flows to thearmature through diode 26 and SCR 32 up to the same phase angle a, afterwhich time current flows through diode 26 and SCR 32 for the remainderof the negative half-cycle.

The capacitor 44 across the armature smooths and thereby improves theform factor of the armature current.

As heretofore explained, the error voltage has a value proportional tothe difference between the speed voltage V and the reference voltage V1f the motor has been running under a constant load and at a constantspeed, and if the load is then increased, the motor will momentarilyslow down, decreasing the speed voltage V and therefore increasing theerror constant and error voltage. The transistor circuit between g-h ofFIG. 6 then acts like a lower resistance; or stated in another way, theresistance of the variable resistor 82 is decreased, and hence the phaseangle a of V is decreased. This causes the SCRs 30 and 32 to tireearlier in their respective half-cycles, so that the motor then receivesmore current and the motor speed is restored.

Similar conditions exist if the speed setting is increased. The increasein reference voltage V causes a momentary increase in error voltage, andthe SCRs fire earlier in their half-cycles, so that the motor cur rentincreases to bring the actual motor speed up to the set speed.

As a result of the conditions in the control circuit, the error voltageE assumes the following values: It is positive when the motor is underload, and it increases as the load increases; that is, under steady loadconditions, E, is small under no load or light load and large heavyload. When the motor speed is higher than the reference speed, the motoracts as a generator, E, being then negative.

DYNAMIC BRAKINC As stated in my above-mentioned patent, if the motor isoperating at a high speed and it is desired to change to a lower speedwhich is done by setting the voltage V to a lower value, the speed willdrop only at a rate determined by the dissipation of kinetic energy ofthe armature and load. This is the generating" or overspeed conditionmentioned above. In order to cause an acceptably fast deceleration,dynamic braking is provided. The dynamic braking system of the presentinvention produces results similar to those of my patent but an improvedcontrol system is herein provided. Under the overspeed condition theerror voltage is negative.

The dynamic braking control circuit is shown as a block 160 in FIG. 1.Across the armature, in series starting with the positive terminal are adiode 162, a dynamic braking SCR 164, and a dynamic braking resis tor166. The SCR 164 is gated on only when the error voltage E,, isnegative, and to this end the voltage E, is fed through lines 170 intoan amplifier 172 (FIG. 9), the output of which is arranged to apply agating pulse to a pulse transformer 173 for gate 168 of the dynamicbraking SCR 164 whenever E, becomes negative. Since the motor is in thegenerating mode at that time, conduction is established from thearmature through the diode 162, SCR 164 and the resistor 166, therebyrap idly dissipating the kinetic energy stored in the arma ture andload,

As the motor slows down it soon reaches a speed at which the errorsignal becomes positive again, thereby gating SCR 164 to off" condition,but the SCR continues to conduct because positive voltage is stillapplied to it from the armature. A commutating SCR 176 is gated on" bythe control circuit at the same time that SCR 164 is gated off. Thiscommutating SCR is connected to the AC line 20 and also to the dynamicbraking SCR 164 in a direction to apply positive line voltage toresistor 166, thus applying reverse voltage to SCR 164, therebycommutating it.

The firing of the commutating SCR 176 occurs when two conditions aresatisfied simultaneously, to wit: l) the error voltage E, is positive,and (2) current is flowing through the dynamic braking SCR 164. Thecircuit for accomplishing this result is shown diagrammatically in FIG,9. The amplifier 172 has E, as an input and delivers an output throughdiode to a line 180 when E, is positive.

As heretofore noted the diode 162 is in series with the dynamic brakingSCR 164 and the resistor 166. When SCR 162 is conducting there is avoltage drop across the diode, which drop is of the order of 0.6 volt.The circuit shown in FIG. 9 includes two amplifiers, namely, theamplifier 172 having a positive output only when the error voltage ispositive, and an amplifier 182 having its input connected across theterminals of the diode 162, so that it has a positive output 184 whenthe positive voltage drop of 0.6 volt exists across the diode. (Thispositive voltage drop cannot exist when the motor is operating in themotor mode, since no current then flows through the dynamic brakingSCR.) An output at 184 is therefore an indication that condition (2)mentioned above exists, namely, that dynamic braking current is flowing.Condition (1), positive error volt age, is indicated by an output at180. Two series transistors 186 and 187 are rendered conducting whenoutputs appear simultaneously on lines 180 and 184. Therefore, when Echanges from negative to positive, the two conditions above mentionedare satisfied, and a pulse is delivered through the two seriestransistors to a pulse transformer 190, the secondary of which isconnected to the gate of the commutating SCR 176.

When SCR 176 is thus gated "on the reverse voltage applied to SCR 164commutates the latter "off," thus terminating the dynamic brakingaction. The AND" control circuit is then no longer operative to maintainthe voltage on the gate of the commutating SCR, and on the next negativehalf-cycle of the AC supply SCR self-commutates to the of condition.These actions of first commutating the dynamic braking SCR and thenself-commutating the commuting SCR are brought about through theconnection of SCR 176 to the AC line, whereby no commutation capacitoror inductor is necessary. Therefore a simple and reliable system isprovided.

MODIFIED ERRoR MEASUREMENT Instead of using a tachometer for generatinga voltage proportional to the speed as in FIG. 3, it is possible toutilize the counter EMF of the armature. The voltage at the armatureterminals may be introduced into the error circuit in place of VHowever, the terminal volt age will be higher than the counter EMF bythe I,,R drop where I, is the armature current and R is the armatureresistance.

To compensate for the l R, drop the circuit in FIG. I0 may be used inplace of that of FIG. 3. A small value compensating resistor 192 is inseries with the armature. A potentiometer 194 is connected across the resistor I92 and the slider thereof is connected with the resistor 78. Thevoltage between the slider and the pos itive end of the compensatingresistor is substantially proportional to the IR drop in the resistor192. Connected from the negative of the armature is a resistor 196 inseries with a resistor 198, the latter being connected to the positiveend of the potentiometer whereby the potential difference across 198 istimes the terminal voltage V The voltage across 192 is small so that Vapproximates the armature terminal voltage V,,. By proper selection ofthe ratio of the resistance and the setting of the potentiometer 194slider, it is possible to compensate for the voltage drop in thearmature resistance, by subtracting the voltage across the positive endand slider of potentiometer 194 from the voltage across R so that theerror voltage E across resistor 78 correctly represents the differencebetween a reference voltage V and a speed voltage V but with the speedvoltage derived from the counter EMF of the motor.

MOTOR CHARACTERISTICS The speed torque characteristics of the motor withfiring phase angle 01 equal to zero are standard for a separatelyexcited motor, that is, with droop in the speed as the torque isincreased. Constant speed operation is by varying the phase angle a inaccordance with the error voltage. For any given speed setting at thevoltage divider 66 the error voltage E, will be such that at any loadthe armature voltage will be maintained at a value on one of thespeed-torque characteristic curves so that the speed will remainconstant.

SUMMARY OF OPERATION Considering the first positive half-cycle, the SCR30 is turned on at a time represented by the phase angle a, so thatcurrent flows to the armature for the remainder of the half-cycle. Atthe end of the positive half-cycle, the SCR 30 turns off by reason ofthe reversal of current. In the ensuing negative half-cycle the bridgesupplies rectified current to the armature circuit during the intervalafter the time represented by the angle a, for the remainder of thehalf-cycle. Therefore the bridge supplies current to the armaturecircuit and filter capacitor during the angle 1r-a and the filtercapacitor supplies current during the angle a of each halfcycle, asindicated by the solid line graph of FIG. 8.

The phase angle at which the SCRs 30 and 32 fire is determined by acomparison of the actual speed with 8 the speed setting. Under lightloads the angle a will be large, and the armature will receive energyfrom the bridge during only a small portion of each half-cycle. Underheavy loads, the angle a will be large, and en ergy will be deliveredfrom the bridge to the armature during most or all of each half-cycle.

In any case the firing angle is determined by the error voltage, whichis the difference between the set voltag and a voltage proportional toactual speed. The sea voltage itself is determined manually for anydesired speed.

The speed-torque characteristics are those of a typical self-excitedmotor. So long as all conditions are constant the motor operates atconstant speed and torque. If the load then increases the speed willtend to fall, and this results in a lower speed voltage and hence ahigher error" voltage, which causes the SCRs to fire at earlier times intheir half-cycles, so that increased energy is supplied to the motor tomaintain its speed under the increase of load. Constant speed operationtherefore involves a shift from one speed'torque characteristic toanother as the load is varied.

Under conditions in which the load is constant, the error voltage willbe constant, and will in all cases be just sufficient to maintain thespeed at the set value. When an increase in speed is called for by a newsetting of the manual control, the error voltage will increase, and thiswill call for firing of the SCRs at earlier points in their cycles, sothat more energy will be supplied to the motor to bring its speed up tothe set value.

Therefore whenever there is an increase of load at a given set speed oran increase of the set speed for a given load, the system automaticallyadjusts itself to increase the energy supplied to the motor. A newequilib rium is established at which the energy supplied to the armatureis just sufficient to maintain the desired speed under the existing loadconditions. The time required for the establishment of the newconditions will be de termined by the time constants of the variouscomponents of the system.

A different situation exists, however, when a reduction of suppliedenergy is called for, as a result of a lowering of the manual set speed.The rotational energy of the armature and load must then be dissipated,and this dissipation of energy might require an inordinately long timeif it depended entirely on friction and windage losses. In order tobring the motor quickly to its new characteristic, the dynamic brakingfeature of the invention is utilized. This operates whenever the errorvoltage is negative. Then the motor is acting as a generator to feedenergy into the dissipation circuit, wherein the excess energy isquickly dissipated electrically in order to bring the motor into the newequilibrium condition in which the energy supplied is just sufficient tomaintain the load at the lower speed.

A feature of the invention is that under all conditions of operation,continuity of armature current is maintained. Therefore, notwithstandingthe sharp establish ment of current at the SCRs 30 and 32, the problemsof poor commutation, cogging and poor regulation frequently encounteredin driving DC motors from rectified current are avoided. Stated inanother way, the form factor of the armature current is maintained nearunity, usually not over about [.05. The form factor is the ratio of rmsto average value. A form factor near unity indicates that nodiscontinuity in the armature current exists.

Another feature of the invention lies in the protec* tion afforded byconstant current operationv Thus overloading or stalling of the motor,or short-circuiting of any part of the d.c. circuit, cannot result indamage to any part of the system. For the same reason, the SCRs areprotected from shoot-through" at all times, rcgardless of transient oroverload conditions in the system. The system is also protected againstdamage from external conditions, such as line voltage transients, andthe semiconductors are protected from shootthrough under all conditions.

I claim:

1. A variable speed motor system comprising an AC source, a DC motorhaving an armature, a bridge rectifier including thyristors, an inductorbetween the source and the rectifier, connections from the rectifier tothe armature, a capacitor across the armature, said thyristorscomprising silicon controlled rectifiers hav ing conduction controlgates. means for applying firing potentials to said gates at selectedtimes in successive half-cycles to control the current supplied to thearmature, a reference circuit, means for introducing into the referencecircuit a manually selected reference voltage, means for introducinginto the reference circuit a speed voltage dependent on the motor speed,means for obtaining in the reference circuit an error voltage which isthe difference between the reference voltage and the speed voltage, aphase control circuit including a capacitor and a variable resistancefor determining the times at which the firing potentials are applied tothe gates in successive half-cycles, and means for controlling thevariable resistance in accordance with the error voltage.

2. A variable speed motor system comprising an AC source, a DC motorhaving an armature, a bridge rectifier including thyristors, an inductorbetween the source and the rectifier, connections from the rectifier tothe armature, a capacitor across the armature, said thyristorscomprising silicon controlled rectifiers hav ing conduction controlgates, and means for applying firing potentials to said gates atselected times in successive half-cycles to control the current suppliedto the armature, a reference circuit, means for introducing into thereference circuit a manually selected reference voltage, means forintroducing into the reference circuit a speed voltage dependent on themotor speed, means for obtaining in the reference circuit an errorvoltage which is the difference between the reference voltage and thespeed voltage, and means controlled by the error voltage for determiningtimes of applying the firing potentials to the gates, a dynamic brakingcircuit connected across the armature and including a dynamic brakingthyristor and a dynamic braking resistor, means for firing the dynamicbraking thyristor when the error voltage is of a value to call for areduction in motor speed, whereby energy is dissipated in the dynamicbraking resistor, a commutating thyristor con nected between the AC lineand the dynamic braking thyristor to apply a reverse voltage to thedynamic braking thyristor, to commutate the latter off, means controlledby the error voltage arriving at a value corresponding to the desiredreduced speed for firing the commutating thyristor, said firingoccurring during a positive half-cycle of the source, the commutatingthyristor being itself commutated off on a succeeding negativehalf-cycle.

3. A variable speed motor system comprising an AC source, a DC motorhaving an armature, a bridge rectifier including thyristors, an inductorbetween the source and the rectifier, connections from the rectifier tothe armature, a capacitor across the armature, said thyristorscomprising silicon controlled rectifiers having conduction controlgates, and means for applying firing potentials to said gates atselected times in successive half-cycles to control the current suppliedto the armature, a reference circuit, means for introduc ing into thereference circuit a manually selected reference voltage, means forintroducing into the reference circuit a speed voltage dependent on themotor speed, means for obtaining in the reference circuit an errorvoltage which is the difference between the reference voltage and thespeed voltage, and means controlled by the error voltage for determiningtimes of applying the firing potentials to the gates, means controlledonly when the error voltage is one polarity to control the times ofapplying the firing potentials to the gates, the error voltage of theopposite polarity occurring when motor is slowing down and is in agenerating condition, a dynamic braking circuit across the armature andincluding a dynamic braking thyristor and a dynamic braking resistor,firing means for applying a gating potential to the dynamic brakingthyristor when the error voltage is of said opposite potential wherebyenergy is dissipated in the dynamic braking resistor, a commutatingthyristor connected between the AC line and the dynamic brakingthyristor to apply a reverse voltage to the dynamic braking resistor tocommutate the latter off, means controlled by conjoint action of theerror voltage resuming its first-named polarity and current flowing inthe dynamic braking circuit to fire the commutating thyristor, andthereby to commutate the dynamic braking thyristor, said commutatingthyristor being itself commutated on a negative half-cycle of the ACline.

4. A system as defined in claim 3, in which the dynamic braking circuitincludes a diode in which there is a forward voltage drop when thedynamic braking thyristor is conducting, and means for utilizing saidvoltage drop to detect the flow of current in the dynamic brakingcircuit and thus to control the firing of the commutating thyristor whenthe error voltage changes from its opposite polarity to its first-namedpolarity.

1. A variable speed motor system comprising an AC source, a DC motorhaving an armature, a bridge rectifier including thyristors, an inductorbetween the source and the rectifier, connections from the rectifier tothe armature, a capacitor across the armature, said thyristorscomprising silicon controlled rectifiers having conduction controlgates, means for applying firing potentials to said gates at selectedtimes in successive half-cycles to control the current supplied to thearmature, a reference circuit, means for introducing into the referencecircuit a manually selected reference voltage, means for introducinginto the reference circuit a speed voltage dependent on the motor speed,means for obtaining in the reference circuit an error voltage which isthe difference between the reference voltage and the speed voltage, aphase control circuit including a capacitor and a variable resistancefor determining the times at which the firing potentials are applied tothe gates in successive half-cycles, and means for controlling thevariable resistance in accordance with the error voltage.
 2. A variablespeed motor system comprising an AC source, a DC motor having anarmature, a bridge rectifier including thyristors, an inductor betweenthe source and the rectifier, connections from the rectifier to thearmature, a capacitor across the armature, said thyristors comprisingsilicon controlled rectifiers having conduction control gates, and meansfor applying firing potentials to said gates at selected times insuccessive half-cycles to control the current supplied to the armature,a reference circuit, means for introducing into the reference circuit amanually selected reference voltage, means for introducing into thereference circuit a speed voltage dependent on the motor speed, meansfor obtaining in the reference circuit an error voltage which is thedifference between the reference voltage and the speed voltage, andmeans controlled by the error voltage for determining times of applyingthe firing potentials to the gates, a dynamic braking circuit connectedacross the armature and including a dynamic braking thyristor and adynamic braking resistor, means for firing the dynamic braking thyristorwhen the error voltage is of a value to call for a reduction in motorspeed, whereby energy is dissipated in thE dynamic braking resistor, acommutating thyristor connected between the AC line and the dynamicbraking thyristor to apply a reverse voltage to the dynamic brakingthyristor, to commutate the latter off, means controlled by the errorvoltage arriving at a value corresponding to the desired reduced speedfor firing the commutating thyristor, said firing occurring during apositive half-cycle of the source, the commutating thyristor beingitself commutated off on a succeeding negative half-cycle.
 3. A variablespeed motor system comprising an AC source, a DC motor having anarmature, a bridge rectifier including thyristors, an inductor betweenthe source and the rectifier, connections from the rectifier to thearmature, a capacitor across the armature, said thyristors comprisingsilicon controlled rectifiers having conduction control gates, and meansfor applying firing potentials to said gates at selected times insuccessive half-cycles to control the current supplied to the armature,a reference circuit, means for introducing into the reference circuit amanually selected reference voltage, means for introducing into thereference circuit a speed voltage dependent on the motor speed, meansfor obtaining in the reference circuit an error voltage which is thedifference between the reference voltage and the speed voltage, andmeans controlled by the error voltage for determining times of applyingthe firing potentials to the gates, means controlled only when the errorvoltage is one polarity to control the times of applying the firingpotentials to the gates, the error voltage of the opposite polarityoccurring when motor is slowing down and is in a generating condition, adynamic braking circuit across the armature and including a dynamicbraking thyristor and a dynamic braking resistor, firing means forapplying a gating potential to the dynamic braking thyristor when theerror voltage is of said opposite potential whereby energy is dissipatedin the dynamic braking resistor, a commutating thyristor connectedbetween the AC line and the dynamic braking thyristor to apply a reversevoltage to the dynamic braking resistor to commutate the latter off,means controlled by conjoint action of the error voltage resuming itsfirst-named polarity and current flowing in the dynamic braking circuitto fire the commutating thyristor, and thereby to commutate the dynamicbraking thyristor, said commutating thyristor being itself commutated ona negative half-cycle of the AC line.
 4. A system as defined in claim 3,in which the dynamic braking circuit includes a diode in which there isa forward voltage drop when the dynamic braking thyristor is conducting,and means for utilizing said voltage drop to detect the flow of currentin the dynamic braking circuit and thus to control the firing of thecommutating thyristor when the error voltage changes from its oppositepolarity to its first-named polarity.